Mylodon harlani, from Rock Creek, Texas. 335 



and might be supposed to have belonged to young individuals. 

 As these animals are, however, monophyodont, it is evident 

 that the only way in which a permanent dentition could be 

 adapted to a growing jaw is by the appearance of but a part of 

 the dentition at a time, or by a conical form of tooth crown, 

 which will permit of the expansion of each tooth at the base. 

 This mode of enlargement does exist for a short time among 

 the Bruta, and this is exhibited in the teeth of the species 

 called Sphenodon by Lund, and Jfegalonyx sphenodonhy myself. 

 The wider base is visible in the inferior series of Mylodon reni- 

 dens in this collection, but the largest dimensions thus obtained 

 are much below those of the M. harlanii. The superior molars 

 display little if any expansion at the basis of the roots. The 

 crowns of both superior and inferior molars are worn. The 

 presence of a third inferior molar of a species of much larger 

 size, but of the same peculiar character as that of the smaller 

 series, shows that the character is constant and significant. 



" The typical specimen consists of almost the entire right 

 maxillary bone, with the teeth in place as above described. 

 The alveolus for the first molar indicates that the latter has a 

 subcylindric shaft, curved slightly posteriorly at the extrem- 

 ities. The second molar has a rather narrowly oval section, 

 with a very slight concavity on the inner side, the long axis 

 directed a little inwards anteriorly if truly antero-posteriorly. 

 Dentine equally thick all round. Extremities of section regu- 

 larly rounded. This tooth is like the corresponding one of the 

 M. harlanii, and with a greater antero-posterior diameter than 

 in the M. robustus. The third molar is reniform in section, 

 wider anteriorly than posteriorly, the long diameter directed a 

 little outwards anteriorly from strictly longitudinal. The pos- 

 terior lobe is narrower than the anterior measured transversely, 

 and neither lobe is flattened or truncate at any point. There 

 are two longitudinal shallow grooves of the shaft ; one subme- 

 dian interior, the other opposite to it, presenting outwards and 

 backwards at an angle of 45° to the axis of the jaw. This tooth 

 is quite different in form from the corresponding one in the 

 M. robustus and M. harlanii, in its relatively much greater 

 antero-posterior diameter, approaching in this respect the form 

 of the second superior molar. . . . 



" Three inferior molars, which correspond in size with the 

 M. renidens, are provisionally referred to it ; especially as the 

 third presents characters quite distinct from those of the 

 M. harlanii. The first tooth is not preserved. The second 

 is like the corresponding tooth in the two larger species. The 

 section is trilobate, the narrowest and most distinct lobe 

 directed posterointernal ly ; the long axis of the two other 

 lobes, which are less distinct from each other, at 45° with that 



